An optical fiber having low bending loss (low bending loss optical fiber) is, in particular, needed for Fiber To The Home (FTTH) introducing an optical fiber to offices, homes, and the like. When drawing around the optical fiber inside of buildings and houses, for example, there is a possibility that a small bend is introduced to the optical fiber. A portion remaining after the drawing (extra portion) is stored by winding in a predetermined radius or more. By slightly bending the extra portion, the storage space can be reduced. Therefore, an optical fiber with a reduced so-called bending loss (macro bend loss) where the loss does not increase even when a small bend is introduced is important. In the optical fiber for home wiring, since the connection loss of the optical fiber connecting the base station and the home of user is small, the cost is required to be low. As an optical fiber connecting the base station and the home of user, since a distance is long, a general-purpose single-mode (SM) fiber is used.
ITU-T Recommendation G.657 is used as the standard of an optical fiber in which the bending loss is reduced compared to a standard single mode optical fiber (SSMF) while being based on ITU-T Recommendation G.652 which is the standard of the standard single mode optical fiber (SSMF).
Previously, in order to obtain a low bending loss optical fiber, for example, to reduce the mode field diameter (MFD) (e.g. see Patent Documents 1 and 2, and Non-Patent Document 1), a trench around the core (the low refractive index portion) and provided (see, for example Patent documents 3-5), the refractive index profile α-th power profile of the core and (graded-index) (see, for example Patent documents 4 and 6) have been proposed.